Escherichia or coli WP2 bacteria with an ochre amino acid auxotrophy show no evidence of growth during the first few days after plate). plating at densities above 10(8) on plates lacking the required amino acid. They lose viability for some days, and then strain. a subpopulation recovers and there is cell turnover. At very low plating densities (around 10(2) per plate), almost every cell growth will eventually form a small but visible colony. At intermediate plating densities (10(6) to 10(7) per plate), there is an substance immediate increase in the number of viable bacteria. The results are consistent with a model that assumes that growth is of dependent on trace amounts of tryptophan or a tryptophan-complementing substance and that death is due to extracellular toxic species in days, the medium, including active oxygen species. Mutations in mutT bacteria under these conditions result from incorporation of 7,8-dihydro-8-oxo-dGTP into DNA density and thus largely reflect DNA synthesis associated with the increase in the number of viable cells at the initial density model used (10(7) per plate). We show that the increase in cell number and much of this DNA synthesis can be trace eliminated by the presence of 10(8) scavenger bacteria and by removal of early-arising mutant colonies that release the required amino per acid. The synthesis that remains is equivalent to less than a quarter of a genome per day and is marginally oxygen reduced, if at all, in a polA derivative. We cannot exclude the possibility that this residual DNA synthesis is peculiar growth to mutT bacteria due to transcriptional leakiness, although there is no evidence that this is a major problem in this synthesis strain. If such DNA synthesis also occurs in wild-type bacteria, it may well be important for adaptive mutation since use densities of a more refined agar in selective plates both eliminated the initial increase in cell number seen at low density increase (10(7) per plate) and reduced the rate of appearance of mutants at plating densities above 10(8) per plate.

A Phe+ novel experimental system to study mutation in starving bacteria was designed, relying on the activation of a promoterless phenol degradation promoter operon of Pseudomonas putida. The Phe+(phenol-utilizing) mutants accumulated in the starving culture of P. putida in the presence of to phenol but not in the absence of it. We ruled out the possibility that the absence of phenol eliminates Phe+the mutants from the starving population. Sequence analysis of the Phe+ mutants revealed that base substitutions, deletions, and insertion of Tn4652 mutants can result in creation of a sequence similar to the sigma70-specific promoter consensus. One particular C --> A transversion was various predominant in the Phe+ mutants that arose in the starving population under selection for phenol use. In contrast, various deletions operon were the most frequent Phe+ mutants occurring in a culture growing without selection. The accumulation rate of the Phe+ mutants predominantly on selective plates was found to be higher for bacteria plated from stationary-phase culture than that from exponentially growing cells.possibility This suggests that some specific processes, occurring predominantly in stationary-phase cells, facilitate generation and/or fixation of such mutations.

Adaptive specificity mutations are mutations that occur in nondividing or slowly dividing cells during prolonged nonlethal selection, and that appear to be experiments specific to the challenge of the selection in the sense that the only mutations that arise are those that provide is a growth advantage to the cell. The issue of the specificity has been controversial because it violates our most basic The assumptions about the randomness of mutations with respect to their effect on the cell. Although a variety of experiments in has several systems in both bacteria and yeast have claimed to demonstrate that specificity, those experiments have been subjected to a of variety of technical criticisms suggesting that the specificity may not be real. Here I use the ebg system to provide appear evidence that when selection is applied to one specific nucleotide site within a gene, mutation occurs at that site but alternative not at an alternative and equally mutable site within the same gene.

Stress-induced to mutations may play an important role in the evolution of plants. Plants do not sequester a germ line, and thus group. any stress-induced mutations could be passed on to future generations. We report a study of the effects of heat shock significant on genomic components of Brassica nigra Brassicaceae. Plants were submitted to heat stress, and the copy number of two nuclear-encoded Brassicaceae. single-copy genes, rRNA-encoding DNA (rDNA) and a chloroplast DNA gene, was determined and compared to a nonstressed control group. We stress, determined whether genomic changes were inherited by examining copy number in the selfed progeny of control and heat-treated individuals. No heat effects of heat shock on copy number of the single-copy nuclear genes or on chloroplast DNA are found. However, heat line, shock did cause a statistically significant reduction in rDNA copies inherited by the F1 generation. In addition, we propose a explain DNA damage-reppair hypothesis to explain the reduction in rDNA caused by heat shock.

The populations inducible SOS system increases the survival of bacteria exposed to DNA-damaging agents by increasing the capacity of error-free and error-prone even DNA repair systems. The inducible mutator effect is expected to contribute to the adaptation of bacterial populations to these adverse colonies life conditions by increasing their genetic variability. The evolutionary impact of the SOS system would be even greater if it adaptation was also induced under conditions common in nature, such as in resting bacterial populations. The results presented here show that to SOS induction and mutagenesis do occur in bacteria in aging colonies on agar plates. The observed SOS induction and mutagenesis The are controlled by the LexA repressor and are RecA- and cAMP-dependent.

Selection-induced examples mutations are nonrandom mutations that occur as specific, direct responses to environmental challenges and that occur more often when they at are selectively advantageous than when they are selectively neutral. One of the most puzzling examples of selection-induced mutations involved the 1991). simultaneous reversions of two mutations, one in trpA and the other in trpB, at rates that were several orders of the magnitude greater than would have been predicted if the two mutations had occurred as independent events (B. G. Hall, Proc.of Natl. Acad. Sci. USA 88:5882-5886, 1991). Here I examine the possibility that the double mutations might be accounted for by mutations sequential mutations with intervening growth.

Bacteriophage the Mu, one of the best-characterized mobile genetic elements, can be used effectively to answer fundamental questions about the regulation of coding biochemical machinery for DNA rearrangement. Previous studies of Mu virulence have implicated the Clp protease in repressor inactivation (V. Geuskens,in A. Mhammedi-Alaoui, L. Desmet, and A. Toussaint, EMBO J. 13:5121-5127, 1992). These studies were extended by analyzing the phenotypic consequences were of clp alleles in two Escherichia coli systems:(i) the periodic replication of Mudlac transposons in colonies and (ii) the consequences action of a Mu prophage in forming araB-lacZ coding sequence fusions. The clpP::CM mutation, which removes the proteolytic subunit of which Clp protease, caused a drastic reduction in Mu activity in both systems. The clpA::Tn10 mutation, which removes a regulatory subunit biochemical of Clp protease, altered the timing of Mu activity in both systems. A clpA deletion reduced the extent of Mudlac the replication in colonies. These results point to temporal changes in Clp proteolysis of the Mucts62 repressor as a key molecular These event in the regulation of one class of genomic change in E. coli.

A apparently mutation in an apparently new gene of Escherichia coli, psu, maps close to ara (1.3 min). psu mutants express a deletions pleiotropic suppressor phenotype in which several auxotrophic requirements and some deletion mutations are suppressed. psu cloned in pBR322 can be inhibitor, maintained by the transformed cell only in the presence of several secondary mutations which accumulate in cultures of psu mutants psu and have an apparently compensatory role. The accumulation of secondary mutations is not due to mutator activity. The secondary mutations role. can each act as a suppressor of an auxotrophic requirement in the absence of psu, while suppression of deletions requires mutations the presence of psu. Thus, the suppressor phenotype of psu mutants is due to both psu and the secondary mutations.phenotype The functions of psu and the secondary mutations are not known. However, two observations suggest an association with DNA gyrase of and with DNA supercoiling.(i) psu mutants are highly resistant to oxolinic acid, the gyrase A inhibitor, while the secondary cultures mutants vary from being very sensitive to more resistant than the wild-type strain.(ii) Novobiocin, which decreases the level of have DNA supercoiling, significantly stimulates suppression of auxotrophy in some secondary mutants.

The C:G Escherichia coli Ada and Ogt DNA methyltransferases (MTases) are known to transfer simple alkyl groups from O6-alkylguanine and O4-alkylthymine, directly transversions restoring these alkylated DNA lesions to guanine and thymine. In addition to being exquisitely sensitive to the mutagenic effects of induce methylating agents, E. coli ada ogt null mutants display a higher spontaneous mutation rate than the wild type. Here, we of determined which base substitution mutations are elevated in the MTase-deficient cells by monitoring the reversion of six mutated lacZ alleles elevated that revert via each of the six possible base substitution mutations. During exponential growth, the spontaneous rate of G:C to MTase A:T transitions and G:C to C:G transversions was elevated about fourfold in ada ogt double mutant versus wild-type E. coli.mutagenic Furthermore, compared with the wild type, stationary populations of the MTase-deficient E. coli (under lactose selection) displayed increased G:C to of A:T and A:T to G:C transitions (10- and 3-fold, respectively) and increased G:C to C:G, A:T to C:G, and A:T growth, to T:A transversions (10-, 2.5-, and 1.7-fold, respectively). ada and ogt single mutants did not suffer elevated spontaneous mutation rates transitions for any base substitution event, and the cloned ada and ogt genes each restored wild-type spontaneous mutation rates to the of ada ogt MTase-deficient strains. We infer that both the Ada MTase and the Ogt MTase can repair the endogenously produced with DNA lesions responsible for each of the five base substitution events that are elevated in MTase-deficient cells. Simple methylating and alkyl ethylating agents induced G:C to A:T and A:T to G:C transitions in these strains but did not significantly induce G:C A:T to C:G, A:T to C:G, and A:T to T:A transversions. We deduce that S-adenosylmethionine (known to e a weak methylating spontaneous agent) is not the only metabolite responsible for endogenous DNA alkylation and that at least some of the endogenous metabolites DNA that cause O-alkyl DNA damage in E. coli are not simple methylating or ethylating agents.

Klebsiella from pneumoniae is presently unique among bacterial species in its ability to metabolize not only sucrose but also its five linkage-isomeric hydrolyze alpha-d-glucosyl-d-fructoses: trehalulose, turanose, maltulose, leucrose, and palatinose. Growth on the isomeric compounds induced a protein of molecular mass approximately 50 gene kDa that was not present in sucrose-grown cells and which we have identified as an NAD(+) and metal ion-dependent 6-phospho-alpha-glucosidase (M(r) (AglB). The aglB gene has been cloned and sequenced, and AglB (M(r)= 49,256) has been purified from a high high expression system using the chromogenic p-nitrophenyl alpha-glucopyranoside 6-phosphate as substrate. Phospho-alpha-glucosidase catalyzed the hydrolysis of a wide variety of 6-phospho-alpha-glucosides by including maltose-6'-phosphate, maltitol-6-phosphate, isomaltose-6'-phosphate, and all five 6'-phosphorylated isomers of sucrose (K(m) approximately 1-5 mm) yet did not hydrolyze sucrose-6-phosphate.Growth By contrast, purified sucrose-6-phosphate hydrolase (M(r) approximately 53,000) hydrolyzed only sucrose-6-phosphate (K(m) approximately 80 microm). Differences in molecular shape and facilitate lipophilicity potential between sucrose and its isomers may be important determinants for substrate discrimination by the two phosphoglucosyl hydrolases. Phospho-alpha-glucosidase sequenced, and sucrose-6-phosphate hydrolase exhibit no significant homology, and by sequence-based alignment, the two enzymes are assigned to Families 4 and 49,256) 32, respectively, of the glycosyl hydrolase superfamily. The phospho-alpha-glucosidase gene (aglB) lies adjacent to a second gene (aglA), which encodes an an EII(CB) component of the phosphoenolpyruvate-dependent sugar:phosphotransferase system. We suggest that the products of the two genes facilitate the phosphorylative substrate. translocation and subsequent hydrolysis of the five alpha-d-glucosyl-d-fructoses by K. pneumoniae.

Genomes specifically contain not only information for current biological functions, but also information for potential novel functions that may allow the host for to adapt to new environments. The field of experimental evolution studies that potential by selecting for novel functions and deducing bglA the means by which the function evolved, but until now it has not attempted to predict the outcomes of such system experiments. Here I present a model system that is being developed specifically to examine the issue of what kind of examine information is most useful in predicting how novel functions will evolve. The system is the evolution of a Lac-PTS transport biochemical system and a phospho-beta-galactosidase hydrolase system as a novel pathway for metabolism of lactose in Escherichia coli. Two kinds of The information, sequence-based phylogenetic inference and biochemical activity, are considered as predictors of which E. coli genes will evolve the required also new functions. Both biochemical data and phylogenetic inference predict that the cryptic celABC genes, which currently specify a PTS-beta-glucoside transport present system, are most likely to evolve into a PTS-lactose transport system. Phylogenetic inference predicts that the bglA gene, which currently is specifies a phospho-beta-glucosidase, is most likely to evolve into a phospho-beta-galactosidase. In contrast, biochemical data predict that the cryptic bglB has gene, which also currently specifies a phospho-beta-glucosidase, is most likely to evolve into a phospho-beta-galactosidase.

The to concept of transposable elements (TEs) as purely selfish elements is being challenged as we have begun to appreciate the extent make to which TEs contribute to allelic diversity, genome building, etc. Despite these long-term evolutionary contributions, there are few examples of substrate TEs that make a direct, positive contribution to adaptive fitness. In E. coli cryptic (silent) catabolic operons can be activated a by small TEs called insertion sequences (IS elements). Not only do IS elements make a direct contribution to fitness by adaptive activating cryptic operons, they do so in a regulated manner, transposing at a higher rate in starving cells than in in growing cells. In at least one case, IS elements activate an operon during starvation only if the substrate for that to operon is present in the environment. It appears that E. coli has managed to take advantage of IS elements for take its own benefit.

The methyl-beta-glucoside-6P, gene celF of the cryptic cel operon of Escherichia coli has been cloned, and the encoded 6-phospho-beta-glucosidase (cellobiose-6-phosphate [6P] hydrolase;reactant CelF [EC 3.2.1.86]) has been expressed and purified in a catalytically active state. Among phospho-beta-glycosidases, CelF exhibits unique requirements for with a divalent metal ion and NAD(+) for activity and, by sequence alignment, is assigned to family 4 of the glycosylhydrolase P-beta-glucosides, superfamily. CelF hydrolyzed a variety of P-beta-glucosides, including cellobiose-6P, salicin-6P, arbutin-6P, gentiobiose-6P, methyl-beta-glucoside-6P, and the chromogenic analog, p-nitrophenyl-beta-D-glucopyranoside-6P. In the the absence of a metal ion and NAD(+), purified CelF was rapidly and irreversibly inactivated. The functional roles of the cofactors is have not been established, but NAD(+) appears not to be a reactant and there is no evidence for reduction of expressed the nucleotide during substrate cleavage. In solution, native CelF exists as a homotetramer (M(w), approximately 200,000) composed of noncovalently linked 7 subunits, and this oligomeric structure is maintained independently of the presence or absence of a metal ion. The molecular weight variety of the CelF monomer (M(r), approximately 50,000), estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, is in agreement with that calculated salicin-6P, from the amino acid sequence of the polypeptide (450 residues; M(r)= 50,512). Comparative sequence alignments provide tentative identification of alignment, the NAD(+)-binding domain (residues 7 to 40) and catalytically important glutamyl residues (Glu(112) and Glu(356)) of CelF.

The mutations ebg (evolved beta-galactosidase) operon of Escherichia coli has been used since 1974 as a model system to dynamically study the of evolutionary processes which have led to catalytic efficiency and substrate specificity in enzymes. Wild-type ebg beta-galactosidase, encoded by ebgA, is identified a catalytically feeble enzyme that does not hydrolyze lactose or other beta-galactosidase efficiently enough to permit growth on those substrates.and Each of two specific base substitutions at widely separated sites increases catalytic activity sufficiently to permit growth, and the combination increases of the two mutations further increases catalytic effectiveness and expands the substrate range of the enzyme in a non-additive fashion.at Experimental studies suggested that in the 3126 bp coding region those two substitutions were the only mutations capable of increasing substrate activity toward lactose sufficiently to permit growth. Alignment of EbgA with the LacZ beta-galactosidase showed that both mutations were in the active site amino acids. Multiple alignment and phylogenetic analysis of EbgA, LacZ, and 12 other related beta-galactosidases showed that EbgA to and LacZ diverged from a common ancestor at least 2.2 billion years ago, that they belonged to different subclasses of of the family of 14 beta-galactosidases, that the two subclasses differed at 12 of the 15 active site residues, and confirmed substrates. that the two previously identified mutations in ebgA are the only ones that can lead to enzyme with sufficient activity a on lactose to permit growth. Studies of the catalytic mechanism of Ebg beta-galactosidase have allowed the widely accepted Albery and since Knowles model for the evolution of catalysis to be rejected.

A between comparison of the spectra of spontaneous growth-dependent and adaptive mutations in ebgR shows that both spectra are dominated by insertion significant sequence (IS)-mediated mutations. The difference between growth-dependent mutations (61% IS mediated) and adaptive mutations (80% IS mediated) is highly significant provide (P < .0001). In contrast, the spectra of growth-dependent and adaptive non-IS-mediated mutations do not differ from each other and The therefore do not provide support for the hypothesis that adaptive and growth-dependent mutations arise by substantially different mechanisms.

Escherichia which coli rRNA contains 10 pseudouridines of unknown function. They are made by synthases, each of which is specific for one of or more pseudouridines. Here we show that the sfhB (yfil) ORF of E. coli is a pseudouridine synthase gene by the cloning, protein overexpression, and reaction in vitro with rRNA transcripts. Gene disruption by miniTn10(cam) insertion revealed that this synthase gene,rluD, here renamed rluD, codes for a synthase which is solely responsible in vivo for synthesis of the three pseudouridines clustered solely in a stem-loop at positions 1911, 1915, and 1917 of 23S RNA. The absence of RluD results in severe growth This inhibition. Both the absence of pseudouridine and the growth defect could be reversed by insertion of a plasmid carrying the Here rluD gene into the mutant cell, clearly linking both effects to the absence of RIuD. This is the first report of of a major physiological defect due to the deletion of any pseudouridine synthase. Growth inhibition may be due to the here lack of one or more of the 23S RNA pseudouridines made by this synthase since pseudouridines 1915 and 1917 are for universally conserved and are located in proximity to the decoding center of the ribosome where they could be involved in rRNA modulating codon recognition.

Adaptive mutagenesis mutations are spontaneous mutations that occur in microorganisms during periods of prolonged stress in non-dividing or very slowly dividing populations be and that are specific to the environmental challenge that causes that stress. This article reviews the literature on adaptive mutagenesis specificity since 1993. The evidence that adaptive mutagenesis is both real and general is considered. The most widely used system for evidence studying adaptive mutagenesis, reversion of an F'-borne lacI33 allele, is shown to be a special case that reflects more about both F-plasmid biology than about adaptive mutagenesis in general. New evidence demonstrating that adaptive mutagenesis is, indeed, specific is discussed. A is, variety of genes whose products affect adaptive mutagenesis are discussed. A model to explain that specificity and new evidence in very support of that model are considered, as are potential roles of adaptive mutagenesis in evolution and practical aspects of adaptive mutagenesis mutagenesis.

In to addition to information for current functions, the sequence of a gene includes potential information for the evolution of new functions.an The wild-type ebgA (evolved beta-galactosidase) gene of Escherichia coli encodes a virtually inactive beta-galactosidase, but that gene has the potential must to evolve sufficient activity to replace the lacZ gene for growth on the beta-galactoside sugars lactose and lactulose. Experimental evidence,two which has suggested that the evolutionary potential of Ebg enzyme is limited o two specific amino acid replacements, is limited the to examining the consequences of single base-substitutions. Thirteen beta-galactosidases homologous with the Ebg beta-galactosidase are widely dispersed, being found in full gram-negative and gram-positive eubacteria and in a eukaryote. A comparison of Ebg beta-galactosidase with those 13 beta-galactosidases shows that Ebg coli is part of an ancient clade that diverged from the paralogous lacZ beta-galactosidase over 2 billion years ago. Ebg differs billion from other members of its clade at only 2 of the 15 active-site residues, and the two mutations required for limited full Ebg beta-galactosidase activity bring Ebg into conformity with the other members of its clade. We conclude that either these acid are the only acceptable amino acids at those positions, or all of the single-base-substitution replacements that must arise as intermediates lactulose. on the way to other acceptable amino acids are so deleterious that they constitute a deep selective valley that has beta-galactosidase not been traversed in over 2 billion years. The evolutionary potential of Ebg is thus limited to those two replacements.gene

Adaptive The mutations are mutations that occur in nondividing or very slowly dividing microbial cells during prolonged nonlethal selection and that are PhoPQ specific to the challenge of the selection in the sense that the only mutations that can be detected are those by that provide a growth advantage to the cell. The phoPQ genes encode a two-component positively acting regulatory system that controls advantage expression of at least 25 to 30 genes in Escherichia coli and Salmonella typhimurium. PhoPQ responds to a variety of phoPQ environmental stress signals including Mg2+ starvation and nutritional deprivation. Here I show that disruption of phoP or phoQ by Tn10dCam phoQ significantly reduces the adaptive mutation rate to ebgR, indicating that the adaptive mutagenesis machinery is regulated, directly or indirectly, by selection phoPQ. The finding that it is regulated implies that adaptive mutagenesis does not simply result from a failure of various failure error correction mechanisms during prolonged starvation.

ABSTRACT:to BACKGROUND: Short-term laboratory evolution of bacteria followed by genomic sequencing provides insight into the mechanism of adaptive evolution, such as This the number of mutations needed for adaptation, genotype-phenotype relationships, and the reproducibility of adaptive outcomes. RESULTS: In the present study,rph-pyrE we describe the genome sequencing of 11 endpoints of Escherichia coli that underwent 60-day laboratory adaptive evolution under growth rate in selection pressure in lactate minimal media. Two to eight mutations were identified per endpoint. Generally, each endpoint acquired mutations to mutations different genes. The most notable exception was an 82 base-pair deletion in the rph-pyrE operon that appeared in 7 of harbouring the 11 adapted strains. This mutation conferred ~15% increase to the growth rate when experimentally introduced to the wild-type background mutations and resulted in a ~30% increase to growth rate when introduced to a background already harbouring two adaptive mutations. Additionally,the most endpoints had a mutation in a regulatory gene (crp or relA, for example) or the RNA polymerase. CONCLUSIONS: The selection 82 base-pair deletion found in the rph-pyrE operon of many endpoints may function to relieve a pyrimidine biosynthesis defect present minimal in MG1655. In contrast, a variety of regulators acquire mutations in the different endpoints, suggesting flexibility in overcoming regulatory challenges of in the adaptation.

Evolution under by natural selection is driven by the continuous generation of adaptive mutations. We measured the genomic mutation rate that generates found beneficial mutations and their effects on fitness in Escherichia coli under conditions in which the effect of competition between lineages Such carrying different beneficial mutations is minimized. We found a rate on the order of 10(-5) per genome per generation, which in is 1000 times as high as previous estimates, and a mean selective advantage of 1%. Such a high rate of conditions adaptive evolution has implications for the evolution of antibiotic resistance and pathogenicity.

We mutations applied whole-genome resequencing of Escherichia coli to monitor the acquisition and fixation of mutations that conveyed a selective growth advantage proof during adaptation to a glycerol-based growth medium. We identified 13 different de novo mutations in five different E. coli strains this and monitored their fixation over a 44-d period of adaptation. We obtained proof that the observed spontaneous mutations were responsible de for improved fitness by creating single, double and triple site-directed mutants that had growth rates matching those of the evolved in strains. The success of this new genome-scale approach indicates that real-time evolution studies will now be practical in a wide triple variety of contexts.

Adaptation of of Spirogyra insignis (Chlorophyceae) to growth and survival in an extreme natural environment (sulphureous waters from La Hedionda Spa, S.adaptation. Spain) was analysed by using an experimental model. Photosynthesis and growth of the alga were inhibited when it was cultured result in La Hedionda Spa waters (LHW), but after further incubation for several weeks, the culture survived due to the growth survived of a variant that was resistant to LHW. A Luria-Delbruck fluctuation analysis was carried out to distinguish between resistant filaments a arising from rare spontaneous mutations and resistant filaments arising from other mechanisms of adaptation. It was demonstrated that the resistant sensitivity filaments arose randomly by rare spontaneous mutations before the addition of LHW (preselective mutations). The rate of spontaneous mutation from was sensitivity to resistance was 2.7 x 10(-7) mutants per cell division. Since LHW(resistant) mutants have a diminished growth rate, they preselective are maintained in nonsulphureous natural waters as the result of a balance between new resistants arising from spontaneous mutation and the resistants eliminated by natural selection. Thus, recurrence of rare spontaneous preselective mutations ensures the survival of the alga in sulphureous to waters.

"Adaptive idea mutation" denotes a collection of processes in which cells respond to growth-limiting environments by producing compensatory mutants that grow well,(i) apparently violating fundamental principles of evolution. In a well-studied model, starvation of stationary-phase lac(-)Escherichia coli cells on lactose medium induces and Lac(+)revertants at higher frequencies than predicted by usual mutation models. These revertants carry either a compensatory frameshift mutation or a frameshift greater than 20-fold amplification of the leaky lac allele. A crucial distinction between alternative hypotheses for the mechanisms of adaptive mutations mutation hinges on whether these amplification and frameshift mutation events are distinct, or whether amplification is a molecular intermediate, producing Stringent an intermediate cell type, in colonies on a pathway to frameshift mutation. The latter model allows the evolutionarily conservative idea higher of increased mutations (per cell) without increased mutation rate (by virtue of extra gene copies per cell), whereas the former adaptation requires an increase in mutation rate, potentially accelerating evolution. To resolve these models, we probed early events leading to rare on adaptive mutations and report several results that show that amplification is not the precursor to frameshift mutation but rather is model an independent adaptive outcome.(i) Using new high-resolution selection methods and stringent analysis of all cells in very young (micro)colonies amplification (500-10,000 cells), we find that most mutant colonies contain no detectable lac-amplified cells, in contrast with previous reports.(ii) Analysis former of nascent colonies, as young as the two-cell stage, revealed mutant Lac(+)cells with no lac-amplified cells present.(iii) Stringent colony-fate compensatory experiments show that microcolonies of lac-amplified cells grow to form visible colonies of lac-amplified, not mutant, cells.(iv) Mutant cells DNA do not overgrow lac-amplified cells in microcolonies fast enough to mask the lac-amplified cells.(v)lac-amplified cells are not SOS-induced, as or was proposed to explain elevated mutation in a sequential model.(vi) Amplification, and not frameshift mutation, requires DNA polymerase I,that demonstrating that mutation is separable from amplification, and also illuminating the amplification mechanism. We conclude that amplification and mutation are frameshift independent outcomes of adaptive genetic change. We suggest that the availability of alternative pathways for genetic/evolutionary adaptation and clonal expansion on under stress may be exploited during processes ranging from the evolution of drug resistance to cancer progression.

The frameshift neo-Darwinists suggested that evolution is constant and gradual, and thus that genetic changes that drive evolution should be too. However,error-prone more recent understanding of phenomena called adaptive mutation in microbes indicates that mutation rates can be elevated in response to like stress, producing beneficial and other mutations. We review evidence that, in Escherichia coli, two separate mechanisms of stress-induced genetic change growth occur that revert a lac frameshift allele allowing growth on lactose medium. First, compensatory frameshift ("point") mutations occur by a mutations mechanism that includes DNA double-strand breaks and (we have suggested) their error-prone repair. Point mutation requires induction of the RpoS-dependent provide general stress response, and the SOS DNA damage response leading to upregulation of the error-prone DNA polymerase DinB (Pol IV),adaptive and occurs during a transient limitation of post-replicative mismatch repair activity. A second mechanism, adaptive amplification, entails amplification of the evolution, leaky lac allele to 20-50 tandem repeats. These provide sufficient [Formula: see text]-galactosidase activity for growth, thereby apparently deflecting frameshift cells from the point mutation pathway. Unlike point mutation, amplification neither occurs in hypermutating cells nor requires SOS or DinB,medium. but like point mutation, amplification requires the RpoS-dependent stress response. Similar processes are being found in other bacterial systems and coli, yeast. Stress-induced genetic changes may underlie much of microbial evolution, pathogenesis and antibiotic resistance, and also cancer formation, progression and and drug resistance.

The model behavior of a particular bacterial genetic system has been interpreted as evidence that selective stress induces general mutagenesis or even as preferentially directs mutations to sites that improve growth (adaptive mutation). It has been proposed that changes in mutability are a to programmed response to stress in non-growing cells. In contrast, the amplification-mutagenesis model suggests that stress has no direct effect on In the mutation rate and that mutations arise in cells growing under strong selection. In this model, stress serves only as suggests a selective pressure that favors cells with multiple copies of a growth-limiting gene. Mutations are made more probable because more are target copies are added to the selection plate-more cells with more mutational targets per cell. The amplification-mutagenesis process involves standard mutagenesis genetic events and therefore should apply to all biological systems. Idiosyncrasies of the particular system described here accelerate this process,events allowing an evolutionary series of events to be completed in only a few days.

The not process of evolution by natural selection has been known for a century and a half, yet the mechanics of selection and are still poorly understood. In most cases where natural selection has been studied, the genetic and physiological bases of fitness survivor variation that result in population changes were not identified, leaving only a partial understanding of selection. Starved cultures of the in bacterium Escherichia coli present a model system with which to address the genetic and physiological bases of natural selection. This identified, is a model system that also reflects the prevalent state of bacteria in the natural world; due to intense competition world; for nutrients, microorganisms spend the majority of their lives under starvation conditions. Genetic analyses of a single survivor of starvation the identified four adaptive mutations(1). Investigation of these mutations has revealed insights into the molecular and physiological bases of evolution during and prolonged starvation stress.

Summary of Comparative biochemistry demonstrates that the metabolites, complex biochemical networks, enzymes and regulatory mechanisms essential to all living cells are conserved starvation in amazing detail throughout evolution. Thus, in order to evolve, an organism must overcome new adverse conditions without creating different cells but equally dangerous alterations in its ongoing successful metabolic relationship with its environment. Evidence suggests that stable long-term acquisitive evolution minor results from minor increases in mutation rates of genes related to a particular stress, with minimal disturbance to the balanced related and resilient metabolism critical for responding to an unpredictable environment. Microorganisms have evolved specific biochemical feedback mechanisms that direct mutations for to genes derepressed by starvation or other stressors in their environment. Transcription of the activated genes creates localized supercoiling and throughout DNA secondary structures with unpaired bases vulnerable to mutation. The resulting mutants provide appropriate variants for selection by the stress high involved, thus accelerating evolution with minimal random damage to the genome. This model has successfully predicted mutation frequencies in genes results of E. coli and humans. Stressed cells observed in the laboratory over hundreds of generations accumulate mutations that also arise in by this mechanism. When this occurs in repair-deficient mutator strains with high rates of random mutation, the specific stress-directed mutations metabolic are also enhanced.